The invention relates generally to the field of heating, ventilating, air conditioning, and refrigeration (HVAC&R) systems, and particularly to systems that can perform heating and cooling functions, such as with chilled water.
A range of systems are known and presently in use for heating and cooling of fluids such as water, brine, air, and so forth. In many building HVAC&R systems, for example, water or brine is heated or cooled and then circulated through the building where it is channeled through air handlers that blow air through heat exchangers to heat or cool the air, depending upon the season and building conditions. Some such systems are designed and used for cooling only, while others may function as a heat pump. In heat pump systems, the direction of refrigerant flow through refrigerant evaporating and condensing heat exchangers is reversed to allow for extraction of heat from a controlled space (cooling mode), or for the injection of heat into the space (heat pump mode).
Existing technologies for heat pump and heat recovery for chilled water systems include several that each benefit from certain advantages, but that also suffer from drawbacks. For example, water-to-water heat pumps generally have good efficiency, and good control over hot water temperatures in heat pump mode. Such systems are generally available, but normally require simultaneous heating and cooling loads for proper operation. They may be prone to fouling if used with wet tower evaporators when used in cooling operation only. Air-cooled chillers with heat recovery are also available, and have the benefits of being inexpensive and efficient at high ambient temperatures. However, such systems have limited control over water temperatures and available heating capacity, particularly at lower ambient temperatures. Air-to-water heat pumps, typically more readily available in Europe and Asia, and less so in North America, offer efficient heating and good control over water temperatures. However, such systems are expensive and do not provide heating and cooling in a single unit. Moreover, pressure drops through a reversing valve used to switch between cooling and heat pump modes are typically very high.
Other heat-pump technologies are available for direct expansion (“DX”) systems where refrigerant directly heats or cools indoor air, but there are issues that limit their application. Air-to-air heat pumps, geothermal heat pumps, and variable refrigerant flow (“VRF”) systems are examples of DX systems. They have obvious limitations for retrofitting to existing buildings with chilled water systems. They are generally useful in smaller buildings or single-story buildings. The sizes of individual systems are small, typically less than 20 tons, so large buildings would require many systems with long runs of refrigerant piping.
An additional issue with these systems is that they can allow refrigerant to leak directly into occupied space, which can create environmental concerns, especially for natural refrigerants. While such concerns exist with current refrigerants, they are clearly more poignant when employing refrigerants with increased flammability and/or toxicity, such as hydrocarbons, ammonia, and HFO-1234yf.
There is a need for improved HVAC&R systems capable of offering both heating and cooling of secondary fluids, such as water or brine.